Supported Platforms
How a Packet Traverses an MPLS Backbone
This section describes how an IP packet is processed as it traverses an MPLS backbone network.
At the entry edge of the MPLS backbone, the IP header is examined by the ingress router or switch. Based on this analysis, the packet is classified, assigned a label, encapsulated in an MPLS header, and forwarded toward the next hop in the LSP. MPLS provides a high degree of flexibility in the way that an IP packet can be assigned to an LSP. For example, in the Junos traffic engineering implementation, all packets arriving at the ingress router or switch that are destined to exit the MPLS domain at the same egress router or switch are forwarded along the same LSP.
Once the packet begins to traverse the LSP, each router or switch uses the label to make the forwarding decision. The MPLS forwarding decision is made independently of the original IP header: the incoming interface and label are used as lookup keys into the MPLS forwarding table. The old label is replaced with a new label, and the packet is forwarded to the next hop along the LSP. This process is repeated at each router or switch in the LSP until the packet reaches the egress routeror switch.
When the packet arrives at the egress router or switch, the label is removed and the packet exits the MPLS domain. The packet is then forwarded based on the destination IP address contained in the packet’s original IP header according to the traditional shortest path calculated by the IP routing protocol.
